1. Field of the Invention
The present invention relates to a linear actuator.
2. Description of the Related Art
There is known a linear actuator for converting rotary movement of a rotor to linear movement of an output shaft to output it (e.g., see JP-A-2005-354858).
The related-art linear actuator includes an output shaft provided with a male threaded portion, and a receiving side provided with a female threaded portion. The male threaded portion is engaged with the female threaded portion to convert the rotary movement to the linear movement in an axial direction. Since the threaded portions structure causes large frictional resistance, so that the efficiency of the structure is poor, the torque of the motor is not effectively transmitted to an output shaft.
A linear motion system of a ball-screw structure is conceivable as a structure for coping with the above problem. In this structure, a ball screw is formed on a shaft, and a ball groove is also formed at a portion corresponding to a nut portion, so that a ball is retained in the ball groove. As a result, the shaft can take a linear movement even by small torque. However, since the helical ball groove has to be formed inside the rotor with high precision, the structure requires high machining cost, so that its price becomes higher.